Tuesday, April 30 th : “A” Day Wednesday, May 1 st : “B” Day Agenda  Homework Questions/Collect  Continue Sec. 7.3: “Formulas & Percentage Composition”  In-Class Assignment: Practice pg. 245: 1-3  Homework: Pg. 61 worksheet: 1 a-e Concept Review Must Show Work!

Homework Questions/Problems  Practice pg. 243: #1-4  Worksheet pg. 56: 1a-d

Molecular Formulas are Multiples of Empirical Formulas  Molecular formula: a chemical formula that shows the number and kinds of atoms in a molecule, but not the arrangement of the atoms.  A molecular formula is a whole-number multiple of the empirical formula.  The molar mass of any compound is equal to the molar mass of the empirical formula times a whole number, n. n (empirical formula) = molecular formula

Comparing Empirical and Molecular Formulas CompoundEmpirical Formula Molecular Formula FormaldehydeCH 2 O Acetic AcidCH 2 OC 2 H 4 O 2 2X the empirical formula n = 2 GlucoseCH 2 OC 6 H 12 O 6 6X the empirical formula n = 6

Rules for Determining a Molecular Formula from an Empirical Formula 1.Use the periodic table to find the molar mass of the empirical formula. 2. Divide the experimental molar mass by the empirical molar mass. (Hint: the bigger # goes on TOP) This number is the multiplier, n. 3. Finally, multiply the empirical formula by the n value to get the molecular formula.

Determining a Molecular Formula from an Empirical Formula (Sample Problem H, pg. 245) The empirical formula for a compound is P 2 O 5. Its experimental molar mass is 284 g/mol. Determine the molecular formula of the compound. 1. Use the periodic table to find the molar mass of the empirical formula: P: 2 (30.97) = g/mol O: 5 (16.00) = g/mol Molar mass of P 2 O 5 = g/mol

Sample Problem H, continued… 2.Find the multiplier, n: n = experimental molar mass of compound molar mass of empirical formula n = 284 g/mol = 2 Hint: the bigger # g/mol always goes on top! 3. To find the molecular formula, simply multiply the empirical formula by 2: 2 (P 2 O 5 ) = P 4 O 10

Example #1  Determine the molecular formula for the following: Molar mass: g/mol Empirical formula: OCNCl 1.Find molar mass of empirical formula: O = g/mol C = g/mol N = g/mol Cl = 35.5 g/mol = g/mol

Additional Practice 2. Find the multiplier, n: n = experimental molar mass of compound molar mass of empirical formula n = g/mol = g/mol 3. To find the molecular formula, simply multiply the empirical formula by 3: 3 (OCNCl) = O 3 C 3 N 3 Cl 3

Example #2  Determine the molecular formula from the following information: Experimental Molar Mass: g/mol Empirical Formula: CH 2 O 1. Find molar mass of empirical formula: C: g/mol H: 2 (1.01 g/mol) = 2.02 g/mol O: g/mol Molar Mass of empirical formula: g/mol

Example #2 cont… 2. Find the multiplier, n: n = experimental molar mass of compound molar mass of empirical formula n = g/mol = g/mol 3.To find the molecular formula, simply multiply the empirical formula by 4: 4 (CH 2 O) = C 4 H 8 O 4

In-Class Assignment/Homework You Must SHOW WORK!  In-Class Assignment: Practice pg. 245: #1-3  Homework: Pg. 61 Worksheet: 1 a-e We will finish this section next time…